docs/system/arm/virt.rst: Add note on CPU features off by default

docs/system/arm/virt.rst: Add note on CPU features off by default

Add a note on CPU features that are off by default in `virt` machines.
Some CPU features will remain off even if a CPU-capable CPU (e.g.,
`-cpu max`) is selected because they require support in both the CPU
itself and in the wider system. Therefore, the user, besides selecting a
CPU that supports such features, must also turn on the feature using a
machine option.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Message-id: 20240122211215.95073-1-gustavo.romero@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>